Combining Two Reaches or Splitting One

Written by Christopher Goodell, P.E., D.WRE
Copyright © The RAS Solution 2015.  All rights reserved.

Suppose you have two separate HEC-RAS rivers and wish to combine them.  There is no “combine two rivers” option in HEC-RAS, but it can be done using the Move Points/Objects option.  Take the very simple reach presented below:



I wish to combine the Upper Tualatin with the Clackamas reach of the Willamette River.  In the figure above, they are disconnected-two separate rivers.  Select Edit…Move Points/Objects and
you’ll notice your cursor change to a pointer with an arrowed cross, and the vertices of the cross section and stream centerlines will highlight.




It’s helpful to zoom in close so you can see the end point nodes and so that you don’t accidentally grab a cross section node. 



Now simply grab the end point of one river and drag it on top of the end point of the other and release the mouse button. 



HEC-RAS will assume you want to put a junction here and will ask you what you want to name it.  It doesn’t matter what you put in for a name, because you will be deleting the junction next.



Make sure to uncheck the option to Move Points/Objects and then select Edit…Delete…Junctions.



Delete the temporary junction and you’re done.  You may need to close and reopen the HEC-RAS geometry schematic to make it look visually correct.



HEC-RAS will automatically take the name of the upstream of the two rivers for the new combined reach.  You can change the name of the river and/or reach using the Edit…Change Name menu item.  It is very important that your river stationing for the two reaches are consistent prior to doing this operation, or it will not work.  River stations must always be in descending order from upstream to downstream.
Now let’s say you want to do the reverse and split a single reach into two reaches.  This two involves a temporary junction.  But to get HEC-RAS to insert a junction, you must draw a temporary tributary river.  You can see in the figure below a new river has been drawn from the upper right to the point on the original river where I want to split it.  Double click the end of the line right on top of the existing river, so that HEC-RAS will recognize you want to create a junction. 



First HEC-RAS will ask you for the name of the new river/reach.  Again, the name doesn’t matter, we’ll be deleting it later. 



Next, HEC-RAS will ask if you wish to split the reach.  Say yes.



Now you have to give a new name for the reach below the split.  I’ll call it Willamette Clackamas, to take it back to the original two reaches I started with.



Now enter a temporary junction name.



If HEC-RAS asks if you want to increase the schematic extents, select Yes.  Now we have a river that was split into two by creating a temporary junction.



Since the temporary river and the temporary junction were only used to split the original river into two, we'll remove them now.  Right click on the junction and select "Disconnect Reaches".  Then go ahead and delete the temporary reaches using Edit…Delete…Reaches.
Now the rivers are disconnected, but they still don’t look like it in the schematic.  You can verify that they are disconnected by checking to see if the new river and reach name show up or by clicking on one and seeing how HEC-RAS highlights it.  Notice here only the lower of the two rivers is highlighted when I click on it.



If you want to visually make them look disconnected, use the Edit…Move Points Objects option again to pull the end points of the streams apart.  Again, it helps to zoom in to do this. 



Uncheck Edit…Move Points/Objects, and we’re done.  If you want it to look exactly like before, you can drag the stream centerline endpoints all the way back to the cross sections, but this is not necessary and has no effect on the computations.